Fast gating photosurface

ABSTRACT

An embodiment of the invention provides a camera comprising a photosurface having a substrate comprising photopixels and associated storage pixels and a controller that controls the photosurface to image a scene by maintaining a bias between the photopixels and their respective storage pixels at all times during an exposure period of the photosurface so that photocharge, substantially upon its generation in a photopixel by light from the scene incident on the photopixel moves towards the photopixel&#39;s storage pixel.

RELATED APPLICATIONS

The current application is a continuation application of U.S.application Ser. No. 12/699,074 filed Feb. 3, 2010, which isincorporated herein by reference.

TECHNICAL FIELD

Embodiments of the invention relate to photosensitive surfaces andmethods of gating such surfaces.

BACKGROUND

Various types of cameras that image a scene to provide distances tofeatures in the scene and thereby capture three dimensions for thefeatures, and for the scene, are well known in the art. Such cameras,often referred to as three-dimensional (3D) cameras includestereoscopic, triangulation, and time of flight, 3D cameras.

Gated, time of flight 3D cameras comprise a photosensitive surface,hereinafter referred to as a “photosurface” and a shutter for shutteringthe camera open and closed. Typically a photosurface comprises aphotosensitive surface of a semiconductor device such as Charge CoupledDevice (CCD) or a Complementary Metal-Oxide-Silicon (CMOS) lightsensitive device, having light sensitive pixels formed on a suitablesubstrate. A shutter used to shutter the camera open or closed maycomprise, by way of example, a gated image intensifier, or a solid stateelectro-optical or acousto-optical modulator.

Shuttering a camera open or closed is also commonly referred to as“gating” a camera open or closed (hence the name “gated time of flight3D camera”) and refers to respectively enabling or preventingregistration by the camera's photosurface of light collected by thecamera. Gating a camera open or closed is also referred to herein asrespectively gating open or gating closed its photosurface. Terms“gating on” and “gating off” a camera or a camera's photosurface arealso used herein and respectively mean gating open and gating closed thephotosurface or camera. “Shuttering” or “gating” a photosurface orcamera without modification by the adverb “open” or “closed” refers toan operation of gating on and/or gating off the photosurface or camera.

A period of time during which a camera is gated open is an exposureperiod during which the camera's photosurface registers light that thecamera collects and directs to the camera's photosurface. A photosurfaceregisters light by accumulating and storing charge, hereinafter“photocharge”, which the light generates in the photosurface's pixels.The light generates photocharge by creating electron-hole pairs in thephotosurface pixels. Depending on a doping configuration of thephotosurface, the accumulated and stored photocharge may be eitherelectrons or holes from the electron-hole pairs.

To image a scene and determine distances from the camera to features inthe scene, the scene can be illuminated, for example with a train oflight pulses radiated from an appropriate light source. Typically, theradiated light pulses are near infrared (NIR) light pulses. The camerais gated open for an exposure period for each radiated light pulse inthe train, following a predetermined delay from a time at which thelight pulse is radiated. For a feature imaged on a pixel in thephotosurface, light reflected by the feature from each radiated lightpulse is registered on a pixel imaging the feature if the reflectedlight reaches the camera during the exposure period following the lightpulse.

Following a last light pulse in the light pulse train, chargesaccumulated in the pixels of the photosurface during the exposureperiods are sensed and converted to voltages. The set of voltagesrepresenting the accumulated charges is referred to as a “frame” of thephotosurface. Since the time elapsed between radiating a light pulse andthe exposure period that follows it is known, a time it took imagedlight that is registered by the pixels to travel from the light sourceto the reflecting features and back to the camera is known. Lightregistered by the pixels, the speed of light and the time elapsed isused to determine distances to features in the scene.

In some gated 3D cameras, only whether or not light is registered on apixel of the 3D camera's photosurface, and the time elapsed betweenlight pulses and exposure periods are used to determine distance fromthe 3D camera to a feature in the scene imaged on the pixel. Because itis unknown when within an exposure period light reflected from a lightpulse by the feature is registered by the gated 3D camera'sphotosurface, accuracy of distance to the feature is typicallydetermined to no better than a distance equal to the speed of lighttimes half a sum of the exposure period duration and light pulse width.

In other gated 3D cameras, an amount of light registered by the pixelduring the exposure periods is also used to determine the distance tothe feature. The amount of registered light is used to indicate whenduring the exposure periods light from the feature is registered, andthereby to improve accuracy of distance measurements to the feature. Fora given and light pulse width and a given exposure period duration,accuracy of distance measurements provided by 3D gated cameras that useamounts of registered light, may be substantially improved relative tothat provided by 3D gated cameras that do not use amounts of registeredlight. Accuracy of distance measurements resulting from use of amountsof registered light may be less than about one tenth of a distance equalto the speed of light times half a sum of the pulse width and durationof the exposure period. In gated 3D cameras in which an amount of lightis used to determine distances to the imaged feature, the amount oflight registered on the pixel is generally corrected for reflectivity ofthe feature, dark current and background light.

Accuracy of measurements made with a gated 3D camera is a function of,among other variables, rise and fall times, jitter and pulse widths ofthe light pulses, and how fast the camera's shutter can gate the cameraopen and closed. A shutter that is sufficiently fast to gate aphotosurface of a 3D camera so that the camera provides distancemeasurements having a required accuracy is referred to as a fastshutter. Typically a fast shutter that is used to shutter a gated 3Dcamera is separate from, and external to the camera's photosurface. Suchan external fast shutter, such as by way of example, a gated imageintensifier noted above, may be capable of being switched betweenoptically blocking and unblocking states in less than a nanosecond (ns)or a few nanoseconds.

It does not appear to be advantageous to gate a 3D gated camera byelectronically turning on and turning off, hereinafter also referred toas “electronically shuttering”, the camera's photosurface. Evenrelatively fast conventional, electronically shuttered interline CCDshave shuttering speeds that are generally in excess of about 100nanoseconds. In addition, a processing time equal to about a microsecondis typically required to acquire a frame of the CCD's pixels. As aresult of the processing time, the CCD cannot be electronicallyshuttered on and off at frequencies greater than or equal to about amegacycle. Because of shuttering speeds in excess of about 100nanoseconds and the limits on gating frequency, conventionally,electronically shuttered interline CCDs are generally too slow for usein a gated 3D camera to determine distances without an external, fastshutter.

It is noted that turning or shuttering on, and turning or shuttering offa photosurface refer respectively to initiating and ending an exposureperiod of the photosurface and not necessarily to initiating orterminating any other function and/or process of the photosurface. Forexample, turning on and turning off a photosurface does not necessarilyrefer to initiating or terminating transfer of photocharge to a readoutamplifier.

To illustrate constraints on shuttering speeds for 3D gated cameras, itis noted that light sources used to provide light pulses for a gated 3Dcamera may not provide sufficient energy in a single light pulse so thatenough light is reflected by features in the scene from the light pulseand back to the camera to provide satisfactory distance measurements tothe features. As a result, as noted above, to acquire distancemeasurements, the scene is exposed to a train of many light pulses. Thecamera is gated open and closed for each light pulse, and light from thelight pulse is registered in pixels of the camera. Intensity of thelight pulses, and their number in the light pulse train, are determinedso that an amount of reflected light registered from all the lightpulses in the train is sufficient to provide acceptable distancemeasurements to features in the scene. As many as a thousand lightpulses might be required in a light pulse train so that an amount ofreflected light that reaches the camera from the scene is sufficient toprovide acceptable distance measurements. To reduce imaging time, and orpossible image blur, to an acceptable level, the light pulse repetitionrate, and corresponding repetition rate of exposure periods, mayadvantageously be as high as at least 10⁷ per second and consequentlyhave a repetition period of about 100 ns or less. Furthermore, lightpulse widths and durations of exposure periods may be equal to about 30ns or less. Conventional electronic shuttering of CCDs is generally muchtoo slow to provide these shuttering speeds and repetition rates.

SUMMARY

An embodiment of the invention provides a camera comprising aphotosurface that is electronically turned on and off to respectivelyinitiate and terminate an exposure period of the camera at a frequencysufficiently high so that the camera can be used to determine distancesto a scene that it images without use of an external fast shutter. In anembodiment of the invention the photosurface comprises pixels formed ona substrate and the photosurface is turned on and turned off bycontrolling voltage to the substrate. In accordance with an embodimentof the invention the substrate pixels comprise light sensitive pixels,hereinafter referred to as “photopixels”, in which light incident on thephotosurface generates photocharge, and storage pixels, which areinsensitive to light that generates photocharge in the photopixels. Inaccordance with an embodiment of the invention, the photosurface iscontrolled so that the storage pixels accumulate and store photochargesubstantially upon its generation during an exposure period of thephotosurface.

Optionally, the photosurface comprises a CMOS photosurface, alsoreferred to simply as a “CMOS”. Optionally, the photosurface comprises aCCD photosurface, also referred to as a “CCD”. Optionally the CCDcomprises an interline CCD.

There is therefore provided in accordance with an embodiment of theinvention, a method of gating a photosurface comprising a substrate onwhich photopixels and associated storage pixels are formed. The methodincludes, applying a first voltage, hereinafter referred to as an “ONvoltage” at a first time to the substrate to turn on the photosurfaceand initiate an exposure period of the photosurface; and applying asecond voltage, hereinafter also referred to as an “OFF voltage” at asecond time to the substrate to turn off the photosurface and end theexposure period.

The method optionally includes biasing the photopixels and associatedstorage pixels so that during the exposure period photocharge generatedin the photopixels by incident light move from the photopixels to, andare accumulated in, the storage pixels substantially upon theirgeneration.

In some embodiments of the invention, the exposure period is less than100 ns. Optionally, the exposure period is less than 70 ns. Optionally,the exposure period is less than 35 ns. Optionally, the exposure periodis less than 20 ns.

In some embodiments of the invention, a frame of the CCD is acquiredfollowing second time.

In some embodiments of the invention the method includes repeatedlyturning on and turning off the photosurface for a plurality of exposureperiods. Optionally, turning on and off the photosurface for theplurality of exposure periods comprises repeatedly turning on andturning off the photosurface at a repetition frequency greater than orequal to 2.5 MHz. Optionally, the repetition frequency is greater thanor equal to 5 MHz. Optionally, the repetition frequency is greater thanor equal to 10 MHz.

In some embodiments of the invention, a frame of the photosurface isacquired for the plurality of exposure periods only followingtermination of a last exposure period of the plurality of exposureperiods.

There is further provided in accordance with an embodiment of theinvention, a method of operating a photosurface comprising a substrateon which photopixels and associated storage pixels are formed. Themethod includes initiating an exposure period of the photosurface; andtransferring photocharge generated in a photopixel by light incident onthe photopixel, from the photopixel to an associated storage pixel priorto terminating the exposure period.

There is further provided in accordance with an embodiment of theinvention, a camera that includes a photosurface having a substrate onwhich photopixels and associated storage pixels are formed and acontroller that controls the photosurface in accordance with anembodiment of the invention to image a scene.

There is further provided in accordance with an embodiment of theinvention, a 3D camera for providing distances to features in a scene.The camera comprises a light source that transmits a train of lightpulses to illuminate the scene and a photosurface having photopixelsthat receive light reflected by the features from the light pulses andgenerate photocharge responsive thereto. The photosurface also hasstorage pixels that accumulate and store photocharge generated in thephotopixels. A controller turns on and turns off the photosurface inaccordance with an embodiment of the invention for each light pulse inthe pulse train responsive to a time at which the light pulse istransmitted.

There is further provided in accordance with an embodiment of theinvention, a method for determining distances to features in a scene.The method includes: transmitting a train of light pulses to illuminatethe scene; turning on and turning off a photosurface in accordance withan embodiment of the invention to register light reflected from thelight pulses by the features; and using the registered light todetermine distances to the features. Optionally, the controller turns onand turns off the photosurface to provide a different exposure period ofthe photosurface for each light pulse in the pulse train responsive to atime at which the light pulse is transmitted.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF FIGURES

Non-limiting examples of embodiments of the invention are describedbelow with reference to figures attached hereto that are listedfollowing this paragraph. Identical structures, elements or parts thatappear in more than one figure are generally labeled with a same numeralin all the figures in which they appear. Dimensions of components andfeatures shown in the figures are chosen for convenience and clarity ofpresentation and are not necessarily shown to scale.

FIGS. 1A and 1B schematically show perspective and cross section viewsrespectively of a portion of a highly simplified interline CCDphotosurface when the CCD is turned on, in accordance with an embodimentof the invention;

FIGS. 2A and 2B schematically show perspective and cross section viewsrespectively of the interline CCD shown in FIGS. 1A and 1B when the CCDis turned off, in accordance with an embodiment of the invention;

FIG. 3 schematically shows a gated 3D camera comprising the CCDphotosurface shown in FIGS. 1A-2B, in accordance with an embodiment ofthe invention;

FIG. 4 shows a time line graph illustrating shuttering the gated 3Dcamera shown in FIG. 3, in accordance with an embodiment of theinvention; and

FIGS. 5A and 5B show a flow diagram of an algorithm for acquiring a 3Dimage of a scene using an interline CCD, in accordance with anembodiment of the invention.

DETAILED DESCRIPTION

An aspect of an embodiment of the invention relates to providing acamera comprising a photosurface that is electronically turned on andturned off to initiate and terminate an exposure period of thephotosurface to light from a scene imaged by the camera by controllingvoltage to the photosurface's substrate. Optionally, the camera is agated 3D camera and voltage to the photosurface substrate is controlledto turn on and turn off the photosurface sufficiently rapidly so thatduration and repetition rates of the photosurface exposure periodsenable the photosurface to be used in the camera to determine distancesto the scene without an external shutter.

In an embodiment of the invention, the photosurface comprises anoptionally different, light insensitive, storage pixel associated witheach light sensitive pixel, hereinafter a “photopixel”. Photochargesgenerated in a photopixel by light from the scene are transferred duringand prior to terminating the exposure period from the photopixel to itsassociated storage pixel, where they are accumulated and stored.Optionally, photocharges generated in a photopixel are transferredcontinuously and rapidly to the photopixel's associated storage pixelduring an exposure period as the photocharges are generated.

In accordance with an embodiment of the invention, following terminationof an exposure period a frame of the photosurface is acquired bytransferring charges stored in the storage pixels to amplifiers forconversion to voltage. In an embodiment of the invention, photocharge isaccumulated and stored in storage pixels during a plurality of exposureperiods, and a photosurface frame is acquired only following terminationof a last of the plurality of exposure periods. In an embodiment of theinvention, a train of light pulse illuminates the scene and exposureperiods of the photosurface are timed responsive to timing of lightpulses in the light pulse train.

In the following detailed description and associated figures, aninterline CCD photosurface in accordance with an embodiment of theinvention is described and schematically shown. A gated 3D camera inaccordance with an embodiment of the invention that comprises theinterline CCD photosurface is also described below, and schematicallyshown in figures associated with the description.

It is noted that whereas the detailed description and associated figuresdescribe and show an interline CCD, practice of embodiments of theinvention is not limited to interline CCD photosurfaces and camerascomprising interline CCD photosurfaces. For example, a photosurface thatoperates in accordance with an embodiment of the invention may have anon-linear structure different from that of an interline CCD and/or maybe based on CMOS technology rather than CCD technology. The photosurfacemay, by way of example, comprise a hexagonal honeycomb structure ofphotopixels and storage pixels such as that described in PCT ApplicationPCT/IB2009/053113 entitled “CMOS Photogate 3D Camera System HavingImproved Charge Sensing Cell and Pixel Geometry” filed on Jul. 17, 2009.

The interline CCD photosurface comprises photopixels and storage pixels.The photopixels are sensitive to light and during an exposure period ofthe photosurface light incident on the photosurface generatesphotocharge in the photopixels. The storage pixels are insensitive tolight, and light incident on the photosurface does not generatephotocharge in the storage pixels. Storage pixels are used to accumulateand store photocharge created in the photopixels during an exposureperiod of the photosurface. Each storage pixel, and optionally eachphotopixel, comprises its own electrode. Functioning of the photopixelsand storage pixels is controlled by controlling voltage to theirrespective electrodes.

In accordance with an embodiment of the invention, the photosurface iselectronically turned on to initiate an exposure period by applying anON voltage to the photosurface substrate and turned off to terminate theexposure period by applying an OFF voltage to the substrate. Thephotopixel and storage pixel electrodes are biased relative to eachother so that when the ON voltage is applied to the interline CCDsubstrate, that is during an exposure period of the CCD, photochargegenerated in a photopixel by light from a scene rapidly transfers to andis accumulated and stored in the photopixel's storage pixel. When theOFF voltage is applied to the substrate, photocharges generated in thephotopixels by light from the scene drain to the substrate, and do nottransfer from the photopixels and accumulate in the storage pixels. Inan embodiment of the invention, the bias of the photopixel electroderelative to the storage pixel electrode is maintained substantially thesame for exposure periods and non-exposure periods of the photosurface.

FIGS. 1A and 1B schematically show perspective and cross section viewsrespectively of a portion of a highly simplified interline CCDphotosurface 20 during an exposure period when the CCD photosurface isturned on by application of an ON voltage to the photosurface substrate,in accordance with an embodiment of the invention. In the figures, thephotosurface is shown exposed to light, represented by wavy arrows 60,from a scene (not shown in FIGS. 1A and 1B).

The figures show CCD photosurface 20 comprising a linear array ofphotopixels and storage pixels characteristic of an interline CCD andaccumulation in the storage pixels of electrons generated by light 60,in accordance with an embodiment of the invention. Interline CCD 20 isassumed, for convenience of presentation, to be configured with a dopingarchitecture so that it registers electrons, hereinafter“photoelectrons”, rather than holes from electron-hole pairs generatedby incident light. In other embodiments, the CCD 20 can be provided witha doping architecture that registers holes from electron-hole pairsgenerated by incident light.

In an example embodiment, the CCD photosurface 20 comprises a siliconp⁺⁺ doped substrate 21, a p doped epitaxial layer 22, and an n dopedlayer 23. Layer 23 is covered with a silicon dioxide insulating layer24. Columns 30 of conductive polysilicon electrodes 31 are formed overregions of the CCD photosurface that comprise photopixels 32 having npjunctions 38. The numeral 30 is used to designate columns of photopixels32 in addition to being used to designate columns of electrodes 31.Columns 40 of optionally polysilicon electrodes 41 are formed overregions of CCD 20 that comprise storage pixels 42 having np junctions48. Columns 40 of storage pixels are optionally overlaid with a“masking” layer 44 of material, optionally a metal, which is opaque tolight 60 and blocks exposure of the regions under storage pixelelectrode 42 to light 60. In some embodiments, electrodes 41 are formedfrom a conducting material that is opaque to light 60 and the electrodesprovide masking of storage pixels 42 in place of masking layer 44, orenhance masking provided by the masking layer. The numeral 40 is used todesignate columns of storage pixels 42 in addition to being used todesignate columns of electrodes 41. Rows (perpendicular to columns 30and 40) of pixels are electrically isolated from each other by fieldoxide insulators 46. Each column 30 of photopixels 32 is associated witha column of storage pixels 42, optionally to its right and iselectrically isolated from column 40 of storage pixels 42 to its left.Isolation of a column 30 of photopixels from the storage column to itsleft can for example be achieved by implanting a suitable dopant, or byforming a shallow trench isolation region, schematically represented byshaded regions 47, between the columns.

In perspective view FIG. 1A and cross section view 1B, CCD substrate 21is electrified to an ON voltage VS_(on) represented by a line labeledVS_(on). In accordance with an embodiment of the invention, electrodes31 located over photopixels 32 are electrified to a voltage V31, whichis more positive than VS_(on), and electrodes 41 over storage pixels 42are electrified to a voltage V41, which is substantially more positivethan voltage V31. Voltages V31 and V41 that are applied respectively tophotopixel electrodes 31 and storage electrodes 41 are represented byline segments labeled with the voltages.

Voltages VS_(on), V31 and V41 back bias rip junctions 38 and 48 underelectrodes 31 and 41 respectively in photopixels 32 and storage pixels42. The voltages generate potential wells in photopixels 32 and storagepixels 42 in CCD 20 that are represented respectively by lines PW32 andPW42. Potential wells PW42 under storage pixel electrodes 41 are deeperthan potential wells PW32 under photopixels 32. Short line segments “PB”represent a potential barrier in the boundary regions between aphotopixel column 30 and an adjacent storage pixel column 40 to theleft. The potential barrier operates to prevent electrons formed in thephotopixel column from drifting to the left and into the left lyingstorage pixel column. The potential barrier is generated by dopantregions 47 noted above.

As a result of the difference in depth of potential wells PW32 and PW42noted above, electric fields are created between a photopixel 32 and itscorresponding storage pixel 42 that drive photoelectrons generated inthe photopixel to the storage pixel. Photoelectrons, that are generatedby light 60 incident on photopixels 32 are represented by shaded circles50 in FIGS. 1A and 1B. Direction of drift of photoelectrons 50 under theinfluence of the fields generated by potential wells PW32 and PW42 isschematically indicated by block arrows 51.

The fields cause photoelectrons 50 to transfer substantially immediatelyupon their creation in a photopixel 32 to the photopixel's associatedstorage pixel 42. A time it takes photocharge to transfer from alocation in the photopixel at which it is generated to the storage pixelis determined by a drift velocity of the photocharge and a distance fromthe location at which it is generated to the storage pixel. The driftvelocity is a function of intensity of fields operating on thephotoelectrons, which intensity is a function of potential differencebetween potential wells PW32 and PW42. For typical potential differencesof a few volts and pixel pitches of less than or equal to about 100microns, photoelectrons transfer to a storage pixel in a time that maybe less than or about equal to a couple of nanoseconds or less than orabout equal to a nanosecond.

Light 60 propagating towards storage pixels 42 does not createphotoelectrons in the storage pixels because the light is blocked fromentering the storage pixels by masking layer 44. As long as voltagesVS_(on), V31 and V41 are configured as discussed above and shown inFIGS. 1A and 1B. CCD interline photosurface 20 is gated on, is in anexposure period and registers light 60 that is incident on itsphotopixels 32. During the exposure period photoelectrons 50 generatedby incident light 60 on a given photopixel 32 are continuously andrapidly transferred from the photopixel and accumulated and stored inthe photopixel's associated storage pixel 42.

FIGS. 2A and 2B schematically show perspective and cross section viewsof interline CCD 20 shown in FIGS. 1A and 1B but with an OFF voltageVS_(off) applied to substrate 21 and the CCD gated off, in accordancewith an embodiment of the invention. OFF voltage VS_(off) is representedby a line labeled VS_(off) in the figures.

VS_(off) is more positive than VS_(on) by a difference ΔVS, which issufficient to forward bias up junctions 38 in photopixels 32 but not npjunctions 48 in storage pixels 42. As a result, whereas potential wellsPW42 in storage pixels 42 may be reduced in depth by the increasedvoltage applied to substrate 21, they remain sufficiently deep tomaintain photocharge they accumulated during the time that the CCD wasgated on by voltage VS_(on). On the other hand, the forward biasing ofnp junctions 38 in photopixels 32 drains charge from the photopixels andphotoelectrons generated by light 60 incident on the pixels stop movingto storage pixels 42, but are attracted to and absorbed in substrate 21.Block arrows 52 in FIGS. 2A and 2B schematically represent motion ofphotoelectrons 50 when CCD 20 is gated off. As long as substrate 21 iselectrified to OFF voltage VS_(off). CCD 20 is gated off andphotoelectrons generated in photopixels 32 by light 60 are not added tostorage pixels 42.

Voltage applied to substrate 21 can be changed from VS_(off) to VS_(on)to rapidly, electronically shutter CCD 20. In particular, the shutteringis sufficiently rapid so that CCD 20 can be electronically gated fastenough for use in a gated 3D camera to measure distances to features ina scene without having to have an additional external fast shutter. Inan embodiment of the invention, the voltage to the substrate is switchedbetween VS_(off) to VS_(on) to gate on the CCD for exposure periodshaving duration, less than or equal to 100 ns. Optionally, the exposureperiods have duration less than or equal to 70 ns. In some embodimentsof the invention, the exposure periods have duration less than 35 ns. Insome embodiments, the exposure periods have duration less than or equalto 20 ns. In an embodiment of the invention, a time lapse betweensuccessive exposure periods is less than or equal to 400 ns and thecamera is gated at a frequency greater than or equal to 2.5 MHz.Optionally, the CCD is gated at a frequency greater than or equal to 5MHz. In some embodiments of the invention the CCD is gated at afrequency greater than or equal to 10 MHz.

By way of a numerical example, the inventors have found that it ispossible to change voltage on the substrate of a Sony® ICX 098 interlineCCD between VS_(off) equal to about 30 volts to VS_(on) equal to about10 volts, and thereby to turn on and turn off the CCD, with a rise timeless than, or about equal to 5 ns.

It is noted that change of voltage applied to a substrate of aphotosurface between an ON voltage and an OFF voltage in accordance withan embodiment of the invention establishes itself throughout thesubstrate with sufficient uniformity and rapidity so that irising isrelatively reduced.

Irising for a photosurface refers to a difference in time it takes for asignal, such as a gating signal, to propagate to and establish itself inall pixels of the photosurface. Ideally, irising should be zero, and allpixels to which a same signal is sent simultaneously should receive thesignal at a same time. Generally, because signals travel at a finitevelocity and photosurfaces have non-zero extent, irising is not zero.For most applications, to an extent that photosurface irising isreduced, the photosurface performs better. For a photosurface comprisedin a gated 3D camera, reduced irising provides for a reduced maximumtime difference, also referred to as “skew”, between times at whichdifferent pixels in the photosurface are gated on or off by a samegating signal. The reduced irising improves accuracy of distancemeasurements provided by the 3D camera.

FIG. 3 schematically shows a gated 3D camera 120 comprising a CCDphotosurface, such as interline CCD 20, being used to measure distancesto a scene 130 having objects schematically represented by objects 131and 132, in accordance with an embodiment of the invention.

Camera 120, which is represented very schematically, comprises a lenssystem, represented by a lens 121, and optionally an interline CCDphotosurface 20 on which the lens system images the scene. Camera 120optionally comprises a suitable light source 126, such as for example, alaser or a LED, or an array of lasers and/or LEDs, that is controllableto illuminate scene 130 with pulses of light. A controller 124 controlspulsing of light source 126 and gating of photosurface 20. Controller124 controls photosurface 20 to be normally off by applying an OFFvoltage VS_(off) to substrate 21 (FIGS. 1A-2B). Timing of light pulsesand gating of CCD photosurface 20 are schematically illustrated alongtime lines 210, 220, 230, 240 in a time line graph 200 shown in FIG. 4.All the time lines have a same, arbitrary, time scale.

To acquire a 3D image of scene 130, controller 124 controls light source126 to emit a train of light pulses, schematically represented by atrain 140 of square “light” pulses 141 having pulse width τ, toilluminate scene 130. A number of light pulses 141 from light pulsetrain 140 are schematically shown along time line 210 in timeline graph200. Light pulses 141 along time line 210 are shown with pulse width τand an overhead arrow pointing left to right to indicate that the lightpulses are emitted towards scene 130.

Following a predetermined time lapse, T, after a time of emission ofeach light pulse 141, controller 124 turns on photosurface 20 to receiveand image light from emitted light pulses 141 reflected by features inscene 130. Controller 124 turns the photosurface on, for exposureperiods having duration optionally equal to light pulse width τ, bylowering voltage applied to substrate 21 by ΔVS from VS_(off) toVS_(on). Timing of voltage VS_(on) applied to substrate 21 to turn onthe photosurface relative to timing of emission of light pulses 141 isschematically shown along time line 220. Voltage VS_(off) is representedby height of a line labeled VS_(off) above time line 220 and voltageVS_(on) is represented by height of lines labeled VS_(on) above timeline 220. Voltages VS_(off), VS_(on), ΔVS, time delay T, and exposureperiod duration τ are shown along the time line. Exposure periods ofphotosurface 20 resulting from changing voltage applied to substrate 21between VS_(off) and VS_(on), are schematically indicated by hatfunctions 231 along time line 230. Hat functions 231 are hereinafterreferred to as “exposure periods 231”.

It is noted that the light pulse width, exposure period duration, anddelay time T define a spatial “imaging slice” of scene 130 bounded by aminimum, lower bound distance, D_(L), and a maximum, upper bounddistance, D_(U), from camera 120. The camera registers light reflectedfrom the scene during exposure periods 231 only for features of thescene located between lower bound distance D_(L) and upper bounddistance D_(U). By way of example, for exposure periods having durationequal to light pulse width τ and delay T, D_(L)=c(T−τ)/2, D_(U)=c(T+τ)/2and the imaging slice has a thickness cτ, where c is the speed of light.

Light from each light pulse 141 reflected by features in scene 130located in the imaging slice is incident on camera 120, collected bylens 121 and imaged on photopixels 32 of CCD 20. Light reflected byfeatures in scene 130 from light pulses 141 is schematically representedby trains 145 of light pulses 146 in FIG. 3 for a few regions of scene130. Amounts of light from reflected pulse trains 145 that are imaged onphotopixels 32 of CCD photosurface 20 and stored in storage pixels ofCCD 20 during exposure periods 231 are used to determine distances tofeatures of scene 130 located in the imaging slice that are imaged onthe photopixels. The distances are used to provide a 3D image of thescene.

Light pulses 146 from a light pulse train 145 that are reflected by aparticular region of scene 130 and imaged on a corresponding photopixel32 in CCD photosurface 20 are schematically shown along time line 240 oftime line graph 200 in FIG. 4. The light pulses are shown with overheadarrows pointing right to left to indicate that the light pulses arereflected light pulse propagating back to gated 3D camera 120.

Temporal overlap of reflected light pulses 146 and exposure periods 231are indicated by shaded regions 150 in light pulses 146 and exposureperiods 231. During temporal overlap, light in light pulses 146 generatephotoelectrons in the corresponding photopixel, which substantiallyimmediately upon their generation drift to and are accumulated instorage pixel 42 (FIGS. 1A-2B) associated with the photopixel.

Following a last emitted light pulse 141 in emitted light pulse train140, controller 124 controls CCD 20 to remain off until all storagepixels 42 in the CCD are read and a frame of the CCD acquired. Readingof the storage pixels and acquiring a frame of the CCD are schematicallyindicated in FIG. 4 by shaded rectangle 250 at the end of the time linesin time line graph 200.

FIGS. 5A and 5B show a flow diagram of an algorithm 300 for acquiring a3D image of a scene using a camera, such as gated 3D camera 120 shown inFIG. 3, comprising an interline CCD, in accordance with an embodiment ofthe invention.

In a block 302 of algorithm 300, the 3D camera is configured to image adesired imaging slice of the scene. As noted above, an imaging slice ofa scene is a portion of the scene between a minimum and maximum distancefrom the 3D camera for which the 3D camera can determine distances. Theminimum and maximum distances are determined by a pulse width of lightpulses used to illuminate the scene and duration of exposure periodsduring which the 3D camera registers light from the scene. Thickness ofthe slice is a difference between the maximum and minimum distances. Asnoted above, for exposure periods having duration equal to the lightpulse width, an imaging slice has a thickness cτ, where c is the speedof light. Configuring the 3D camera generally comprises aiming, focusingand determining a depth of field for the camera to match the location,thickness, and an angular extent of the desired imaging slice.

In a block 304, operating parameters of the 3D camera that configureilluminating the scene with a train of light pulses and gating the 3Dcamera are initialized to match the geometry and location of the imagingslice. The operating parameters optionally comprise a number N of lightpulses in the pulse train, a light pulse width τ, pulse repetitionfrequency f, a delay time T at which the CCD is gated on following atime at which each light pulse in the pulse train is emitted, andduration of an exposure period, “τ_(g)”. Optionally, the exposure periodduration τ_(g) equals the light pulse width τ. In an embodiment of theinvention, a counting index n, and time t of a system clock optionallycomprised in a controller (e.g. controller 124, FIG. 3) of the 3Dcamera, are set to zero. A light pulse transmission time, t_(x) is setto a time t_(xo) and the system clock is turned on. The time t_(xo) is atime at which a first light pulse is transmitted to illuminate the sceneafter the system clock is turned on.

In a block 306, in accordance with algorithm 300, the controller setsvoltage to the substrate of the CCD to OFF voltage V_(off) so that theCCD is off and does not accumulate photocharge generated by light fromthe scene that is incident on the photosurface. In a block 308, thecontroller increases counter index n by 1 and in a decision block 310the controller compares clock time t to t_(x), t is not equal to t_(x),the controller reverts to block 310 to again compare t to t_(x). If timet=t_(x), in a block 312, the controller controls a light source (e.g.light source 126, FIG. 3) to transmit a first light pulse having pulsewidth τ to illuminate the scene. Following transmission of the lightpulse, in a decision block 314, the controller compares system clocktime t to (t_(x)+T). If t is not equal to (t+T) the controller revertsto decision block 314 to again compare system clock time t to (t_(x)+T).If system clock time t=(t_(x)+T), in a block 316, the controller appliesvoltage V_(on) to the substrate of the CCD to gate on the CCD andaccumulate photocharge generated by light from the scene in storagepixels of the CCD.

After gating the CCD on, in a block 318, the controller compares systemclock time t to (t_(x)+T+τ_(g)). If t is not equal to (t_(x)+T+τ_(g)),the controller reverts to block 318 to again compare t to(t_(x)+T+τ_(g)). If t=(t_(x)+T+τ_(g)), the controller proceeds to ablock 320 shown in FIG. 5B. In block 320 the controller changes voltageon the substrate of the CCD to V_(off) to gate off the CCD. In a block322, transmission time t_(x) is increased by a time 1/f to set a time totransmit a next light pulse to illuminate the scene.

In a decision block 326 index n is compared to the total number of lightpulses N that are to be transmitted to illuminate the scene. If n<N thecontroller returns to block 308 (FIG. 5A) to increase n by one, transmitanother light pulse to illuminate the scene and cycle through blocks 310to 324. If on the other hand, n is not less than N, a frame of the CCDis acquired in a block 326 and in a block 328, data provided by theframe is processed to provide a 3D image of the scene.

In the description and claims of the present application, each of theverbs, “comprise” “include” and “have”, and conjugates thereof, are usedto indicate that the object or objects of the verb are not necessarily acomplete listing of components, elements or parts of the subject orsubjects of the verb.

Descriptions of embodiments of the invention in the present applicationare provided by way of example and are not intended to limit the scopeof the invention. The described embodiments comprise different features,not all of which are required in all embodiments of the invention. Someembodiments utilize only some of the features or possible combinationsof the features. Variations of embodiments of the invention that aredescribed, and embodiments of the invention comprising differentcombinations of features noted in the described embodiments, will occurto persons of the art. The scope of the invention is limited only by theclaims.

The invention claimed is:
 1. A camera comprising: a photosurface havinga substrate comprising photopixels and associated storage pixels; and acontroller operable to control the photosurface to image a scene by:applying voltage to the substrate at first and second times torespectively initiate and terminate an exposure period of thephotosurface during which light from the scene is incident on thephotosurface; and biasing the photopixels and their respective storagepixels to maintain a photocharge transfer state substantially at alltimes between the first and second times in which photocharge,substantially upon its generation in a photopixel, moves towards thephotopixel's storage pixel.
 2. The camera according to claim 1 andcomprising a light source configured to transmit a train of light pulsesto illuminate the scene, wherein the controller is operable to initiateand terminate an exposure period of the photosurface for each lightpulse in the pulse train responsive to a time at which the light pulseis transmitted.
 3. The camera according to claim 2 wherein thecontroller is operable to maintain the bias between the photopixels andtheir respective storage pixels between exposure periods substantiallythe same as the bias during the plurality of exposure periods.
 4. Thecamera according to claim 2 wherein the controller is operable toacquire a frame of the photosurface after the exposure period followinga last light pulse in the train of light pulses.
 5. The camera accordingto claim 2 wherein the light source is configured to transmit lightpulses in the pulse train at a repetition frequency greater than orequal to 2.5 MHz.
 6. The camera according to claim 5 wherein therepetition frequency is greater than or equal to 5 MHz.
 7. The cameraaccording to claim 6 wherein the repetition frequency is greater than orequal to 10 MHz.
 8. The camera according to claim 1 wherein a time lapsebetween the first and second times is less than 100 ns.
 9. The cameraaccording to claim 8 wherein the time lapse between the first and secondtimes is less than 35 ns.
 10. The camera according to claim 9 whereinthe time lapse between the first and second times is less than 20 ns.11. The camera according to claim 2 wherein the controller usesphotocharge accumulated in a storage pixel during the exposure periodsto determine a distance to a feature in the scene imaged on thephotopixel associated with the storage pixel.
 12. The camera accordingto claim 1 wherein the photosurface comprises at least one photosurfacechosen from the group consisting of a CCD, a CMOS, and an interlinephotosurface.